es advantage of
the instincts of others;--that the canon in natural history, of "natura
non facit saltum" is applicable to instincts as well as to corporeal
structure, and is plainly explicable on the foregoing views, but is
otherwise inexplicable,--all tend to corroborate the theory of natural
selection.

This theory is, also, strengthened by some few other facts in regard
to instincts; as by that common case of closely allied, but certainly
distinct, species, when inhabiting distant parts of the world and living
under considerably different conditions of life, yet often retaining
nearly the same instincts. For instance, we can understand on the
principle of inheritance, how it is that the thrush of South America
lines its nest with mud, in the same peculiar manner as does our British
thrush: how it is that the male wrens (Troglodytes) of North America,
build "cock-nests," to roost in, like the males of our distinct
Kitty-wrens,--a habit wholly unlike that of any other known bird.
Finally, it may not be a logical deduction, but to my imagination it
is far more satisfactory to look at such instincts as the young cuckoo
ejecting its foster-brothers,--ants making slaves,--the larvae of
ichneumonidae feeding within the live bodies of caterpillars,--not as
specially endowed or created instincts, but as small consequences of one
general law, leading to the advancement of all organic beings, namely,
multiply, vary, let the strongest live and the weakest die.

8. HYBRIDISM.

Distinction between the sterility of first crosses and of hybrids.
Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication. Laws governing the sterility
of hybrids. Sterility not a special endowment, but incidental on other
differences. Causes of the sterility of first crosses and of hybrids.
Parallelism between the effects of changed conditions of life and
crossing. Fertility of varieties when crossed and of their mongrel
offspring not universal. Hybrids and mongrels compared independently of
their fertility. Summary.

The view generally entertained by naturalists is that species, when
intercrossed, have been specially endowed with the quality of sterility,
in order to prevent the confusion of all organic forms. This view
certainly seems at first probable, for species within the same country
could hardly have kept distinct had they been capable of crossing
freely. The importance of the fact that hyb